I continue to get the system error quite often. The checking miles upgrade option has never worked on any of my itineraries. Reporting this to MP, who asks for screenshots over and over, is a waste of time.

Once a booking encounters the error, they can't fix it. Many times I have just cancelled the booking over the phone and made a new booking (unrestricted tickets) so that the new booking can be accessed, albeit not checking for miles upgrade, but for other purposes such as entering passport info.

I have no idea how CX can continue to run the complexity of a large airline with such poor IT resources.